Compile and run the program below with

```
ocamlfind opt -linkpkg -package lwt.unix foo.ml && ./a.out
```

As written with `Lwt.pick`, `p2` is canceled first, its callback (registered by `Lwt.catch`) runs first, and the output is

```
p2's callback sees p3 pending
p3's callback sees p2 failed
```

If changed to have `Lwt.npick` from the PR, `p3` is completed first, its callback (registered by `Lwt.bind`) runs first, and the output is

```
p3's callback sees p2 pending
p2's callback sees p3 resolved
```

Ideally, changing `pick` to `npick` shouldn't produce this difference in callback execution order and seen states, as it makes refactoring relatively more dangerous.